1. Field of the Invention
The present invention relates to the process for recovering toluene diisocyanate (TDI) monomers from TDI residues, and the process for preparing new stable polyisocyanate materials by the mixing the TDI residues with MDI and/or MDI oligomers followed by removing the TDI monomers.
2. Brief Description of the Prior Art
It is well known in the art that distillation residues of toluene diisocyanates from the phosgenation products thereof are difficult to handle. During the phosgenation of toluene diamine to toluene diisocyanate followed by distillation of TDI, there are formed relatively high molecular weight by-products containing uretdione, isocyanurate, carbodiimide, uretone imine, urea and/or biuret groups. Depending upon the content of o-toluene diamine starting material, there can also be formed methylbenzimidazoles during phosgenation and with time, they are biuretized with the free isocyanate groups present, with an accompanying formation of crosslinked products. Much work has been done in the art to improve on the process of treating toluene diisocyanate residues in order to make them more useful.
One of the art-known processes for treating toluene diisocyanate residues comprises recycling of the toluene diisocyanate residues and hydrolyzing the residues. Only small amounts of the toluene diisocyanate are converted to toluene diamine. See U.S. Pat. Nos. 3,128,310, 3,331,876 and 4,311,800.
Another proposed process comprises dissolving toluene diisocyanate residues which still have a considerable content of free isocyanate groups (greater than 20% by weight) in an organic solvent in the presence of monomeric diisocyanates, optionally at a high temperature, and for using these residue solutions as the isocyanate component in the polyisocyanate addition process. In practice, however, this process fails because of the inadequate stability of the solution during storage and inability to standardize the solution. See U.S. Pat. Nos. 3,364,361 and 3,455,836.
Yet another process for toluene diisocyanate recovery comprises processing the residue in a fluidized bed at 140 to 280 degrees Centigrade. The remaining residue is of a dust-like consistency and, as such, cannot be further utilized. See DE-OS 2,915,830 and DD-PS 130,142 or DE-OS 2,452,804.
A somewhat related process has been proposed for recovery of the isocyanate components from the toluene diisocyanate residue by introducing other polyisocyanates such as methylene diphenyl diisocyanate into the residue and subsequently distilling the resulting product, and optionally post-treating the resulting product. See DD-257,827-A1 which describes a process by which the distillation residue from toluene diisocyanate production is distilled after mixing with diphenyl methane diisocyanate or its higher ring homologs. Toluene diisocyanate is recovered as a distillate and the sump product is also recovered as an additional isocyanate component. Also see application Ser. No. 07/928,561 filed on Aug. 11, 1992.
The present invention provides an improved process for the recovery of toluene diisocyanate monomer from toluene diisocyanate residue solutions wherein the remaining material is useful as an isocyanate.